You're not going to use or waste that much water washing your RV. If you want to be "green", put a five gallon bucket in your shower and let it fill while you're waiting for hot water. After a few showers it will be full. Use it as your wash water and then rinse with your garden hose sparingly. If you buy one of those spray nozzles with the 24" wand, you can reach higher and rinse downward using less water.
I wash my cars at the drive thru car wash that recycles, but I still wash my coach at home. The rules say you must use a hose with a nozzle that shuts off when released. I felt bad about using water until I drove through the central valley and saw all the farmers and businesses using water like it was free and never ending.